Freshwater biodiversity is threatened by growing human consumption and contamination of fresh water - a globally scarce resource. As human populations increase, the quality and quantity available for freshwater biodiversity declines. The result is a tragedy of the freshwater commons with increasing competition among groups of humans – evident from the hydropolitics of transboundary rivers - and between humans and nature. Humans may even be approaching the planetary boundary for freshwater use. Pollution and contamination are widespread, with emerging threats from microplastics and pharmaceuticals. Dams, drainage-basin disturbance, climate change, alien species, and overexploitation of aquatic animals pose additional threats. Their synergistic effects are evident from a global analysis of rivers: both biodiversity and human water security are at risk in many parts of the world while, in others, investments in infrastructure have enhanced water security although biodiversity remains under threat. Everywhere on Earth where there are substantial human populations, freshwater biodiversity is threatened. In many of these places, human water security is at risk also.